In the course of manufacturing automobile parts, construction machine parts and industrial machinery parts, such as axle shafts, drive shafts, outer races for constant velocity joints, or gears for power transmission, those components are generally subjected to a surface hardening treatment or hardened by normal “quenching and tempering” in order to provide a desired mechanical property after being machined into a prescribed shape.
Particularly for automotive parts among the above described components, there has been an increased demand for reducing size and weight in order to address environmental challenges such as improving the fuel economy and reducing the exhaust emissions of automobiles in recent years. As a result, loading on the components has been more and more increasing and, in particular, it has become important to improve the fatigue strength in the “low to medium cycle region” where impulsive loading is problematic.
In order to enhance the fatigue strength of components, generally, “carburizing and quenching” is often used as a surface hardening treatment.
However, in the case of a normal “carburizing and quenching” treatment, the carbon concentration of the hardened portion of the surface reaches about 0.8% by mass %, and the micro structure after quenching becomes a high-carbon martensite structure. As a result, although a high hardness can be achieved, it is difficult to avoid “embrittlement” caused by the high-carbon martensite structure.
“Martensite” in the present description refers to a structure having a “lath-type structural form” among a so-called “fresh martensite” and a “self-tempered martensite”, which are obtained by an isothermal transformation and a continuous cooling transformation, and “tempered martensite” which is obtained by tempering the same, and it also includes a structure in which carbides such as ε or θ carbide is precipitated in the above described “lath-type structure”.
Even in the case of tempering the above described “fresh martensite” and “self-tempered martensite”, if tempering is carried out at a high temperature, for example at more than 700° C., which causes the “lath-type structure” to recrystallize to form an equiaxed ferrite, it will not be included in the “tempered martensite”.
Non Patent Document 1 describes a study of materials which are assumed to be subjected to a “carburizing and quenching” treatment. However, only with such modification of the material, it is difficult to avoid “embrittlement” caused by the above described high-carbon martensite structure. Thus, it is not sufficient to improve the fatigue strength in the “low to medium cycle region” associated with impulsive loading.
Accordingly, a method of achieving high fatigue strength that is being studied is a method of performing a shot peening treatment after a surface hardening treatment such as carburizing and quenching to provide compressive residual stress on the component's surface. Specifically, for example, Patent Documents 1 to 4 propose a high-fatigue-strength component and a manufacturing method thereof, which combines a surface hardening treatment such as carburizing and quenching, etc. and a shot peening treatment. Patent Document 5 proposes a high-fatigue-strength component and a manufacturing method thereof as another method of achieving high fatigue strength, in which after the surface hardening treatment by carburizing and quenching is carried out, an induction hardening is further performed on a particular location of the product.
That is, Patent Document 1 discloses a “method for manufacturing driving system machine parts having high fatigue strength”, wherein steel containing 0.1 to 0.3% of carbon is prepared and shaped into a machine part, and the machine part is subjected to a carburizing or carbonitriding treatment so as to allow a slack quenched layer having a Vickers hardness of not less than 400 and less than 700 to be present in a range of depth from not less than 10 μm to not more than 50 μm from the surface; or wherein steel containing 0.35 to 0.75% of carbon is prepared and shaped into a machine part, and the machine part is subjected to quenching so as to allow a slack quenched layer having a Vickers hardness of not less than 400 and less than 700 to be present in a range of depth from not less than 10 μm to not more than 50 μm from the surface, and further to tempering; and wherein thereafter the machine part is subjected to a shot peening treatment by use of peening media having a hardness of not less than 500 in Vickers hardness.
Patent Document 2 discloses a “production method of carburization hardened product having high fatigue strength” comprising: preparing a steel material which contains, by mass %, C: 0.1 to 0.4%, Si; not more than 0.3%, and Al: 0.02 to 0.08%, also contains two or more kinds of elements selected from a group consisting of Mn; 0.3 to 3.1%, Ni; 0 to 6%, Cr: 0 to 1.2%, and Mo: 0 to 1.2% so as to satisfy [6.4%≦2[Mn]+[Ni]+[Cr]+[Mo]≦8.2%], and further contains, as desired, one or more kinds selected from a group consisting of Nb: 0.005 to 0.2% and V; 0.03 to 0.8%, with the balance being iron and unavoidable impurities; subjecting the steel material to a carburizing or carbonitriding treatment such as one which satisfies [0.55%≦surface carbon content (mass %)+surface nitrogen content (mass %)≦0.90%], and then to a quenching from a single austenite phase region, to obtain steel material whose hardened layer by carburizing and quenching has a maximum hardness of 550 to 620 in Vickers hardness, and wherein an area fraction of retained austenite in a region from the surface to a depth of 300 μm is never be not more than 20%; and thereafter subjecting the steel material to a shot peening treatment under the condition that an arc height is not less than 0.6 mmA.
Patent Document 3 discloses a “component for high interface pressure having excellent pitting resistance and wear resistance”, wherein the component is made up of steel containing, by mass %, C: 0.15 to 0.60%, Si: 0.01 to 2.00%, Mn: 0.01 to 2.00%, Al: 0.003 to 0.050%, N: 0.005 to 0.100%, Cr: 1.50 to 6.00%, and Mo: 0.01 to 3.00%, satisfying Cr+2Mo: 2.00 to 8.00%, the steel further containing, as desired, one or two kinds selected from Ni: 0.1 to 2.0%, B: 0.0001 to 0.0020%, V: 0.01 to 0.50%, Nb: 0.01 to 0.20%, and Ti: 0.01 to 0.20%, with the balance being Fe and unavoidable impurities, and wherein an area fraction of carbides is not more than 2% in a surface layer, where the square root of the product of the major axis and the minor axis of each carbide is not less than 2 μm; and a “production method of component for high interface pressure” wherein a carburizing, quenching, and tempering treatment, or a carbonitriding, quenching, and tempering treatment are carried out by controlling such that a heating temperature is 930 to 1050° C., a carbon concentration of carburized surface layer is 0.60 to 0.80%, and a quenching temperature is 850 to 900° C.; or after the above described tempering treatment is carried out, at least any one of surface hardening treatments including grinding, shot peening, hard shot peening, and fine particle shot peening is performed.
Patent Document 4 discloses a “carburized component superior in low cycle fatigue characteristic” wherein the component contains, by mass %, C: 0.10 to less than 0.30%, Si: not more than 0.10%, Mn: 0.20 to 0.60%, P: not more than 0.015%, S: not more than 0.035%, Cr: 0.50 to 1.00%, Mo: 0.50 to 1.00%, B: 0.0005 to 0.0030%, Ti: 0.010 to 0.100%, Nb: 0.010 to 0.100%, with the balance made up of Fe with unavoidable impurities, and wherein a surface layer C concentration after a gas carburizing treatment is 0.40 to 0.60%, an effective hardened layer depth, with a critical hardness being 513 in Vickers hardness, is 0.6 to 1.2 mm, and a surface hardness after a shot peening treatment is not less than 700 in Vickers hardness.
Patent Document 5 discloses a “production method of case-hardened product having high-fatigue strength” comprising: processing a steel material into a desired product shape, the steel material containing, by mass ratio, C: 0.15 to 0.35%, Al: 0.01 to 0.15%, N: 0.005 to 0.025%, Mn: 0.30 to 1.2%, Cr: 0.30 to 1.20%, and S: 0.01 to 0.20% and, as desired, further containing one element or two elements in combination out of two groups: (a) Nb: 0.020 to 0.120% and Ti: 0.005 to 0.10%, and (b) Mo: not more than 1.0%, Ni: not more than 4.0%, Cu: not more than 2.0%, and V: not more than 1.0%, with limitations of P: not more than 0.01% and Si: not more than 0.50%, and the balance being Fe and unavoidable impurities; subjecting the product to carburizing and quenching with a carbon potential at which a carbon potential Cp is in a range of 0.4 to 0.9 by mass %, and the difference between the carbon potential and the carbon concentration of the material is not less than 0.2 mass %; and thereafter subjecting one part or whole part of the product to an induction hardening by which a depth of 0.3 to 1.5 times the whole hardened layer at the time of carburizing is austenitized.